The present invention relates to ultrasound systems and, in particular, an improved system and method for contrast agent imaging.
In ultrasound imaging, ultrasonic waves are transmitted into a body from the surface of the skin and are reflected from tissues and cells within the body. The reflected echoes are received by an ultrasonic transducer and processed to produce an image or measurement of blood flow.
Materials known as xe2x80x9cultrasonic contrast agentsxe2x80x9d can be introduced into the body to enhance ultrasonic diagnosis. Contrast agents are substances which will strongly interact with ultrasonic waves, returning echoes which may be clearly distinguished from those returned by blood or tissue. One class of substances which has been found to be useful in an ultrasonic contrast agent is gas, in the form of microbubbles. Microbubbles present a significant acoustic impedance mismatch in comparison to tissue and fluids and nonlinear behavior in certain acoustic fields which is readily detectable through special ultrasonic processing.
Certain microbubble contrast agents exhibit significant detectable nonlinear responses at frequencies other than the transmitted ultrasound frequency. This property is useful for clutter rejection of the received signals. When the transmitted frequency band is used as the receive frequency band, echoes will be returned from the microbubbles, but also from the surrounding tissue, the latter as clutter in the received echo signals. But with contrast agents, significant return energy is concentrated outside the fundamental transmit frequency band, so that fundamental band clutter from tissue can be ignored. Since tissue generally reflects relatively minimal energy outside the fundamental band, the received energy outside the fundamental band enables the microbubble echoes to be received with a high signal to noise ratio.
A useful technique for contrast enhancement is known as phase inversion. According to one implementation of this technique, first and second ultrasound pulses are alternatively transmitted into the specimen being imaged. The pulses are amplitude modulated signals; the first pulse differs from the second by the phase, for example, by 180 degrees. The echo signals generated are measured and combined. Echoes generated by linear media cancel, but will not cancel if an echo results from a nonlinear medium. Further details regarding this technique are described in commonly-assigned U.S. Pat. No. 5,632,277, titled xe2x80x9cUltrasonic Imaging System Employing Phase Inversion Subtraction to Enhance the Image.xe2x80x9d
The wideband phase inversion scheme receives harmonic signals along with fundamental residuals, flow motion, and non-symmetrical bubble responses to compression and rarefaction pulses.
This technique is illustrated in greater detail in FIGS. 1 and 2. More particularly, FIG. 1 is a graph illustrating the spectrum 101 of contrast agent flow using phase inversion wideband imaging. A wideband receiving filter 102 is positioned at 2.0 MHz to 6 MHz, where 2.0 MHz is the first harmonic. It is noted that these frequency values are exemplary only. As can be seen, a substantial portion of the contrast agent flow spectrum falls within the wideband filter, i.e., the second harmonic and partial fundamental residuals.
FIG. 2 illustrates the corresponding tissue spectrum 103, with the same wideband filter 102. It is known that the phase inversion imaging scheme achieves 25-30 dB fundamental cancellations in tissue regions. As a result, tissue second harmonic signals are stronger than fundamental residuals. Thus, in tissue regions, the spectra of phase inversion summation signals exhibit double humps within the band of the wideband filter. As such, while the wideband filter is beneficial to contrast flow enhancement, it degrades both image spatial resolution and contrast resolution in tissue regions.
As such, there is a need for a system having improved image quality in tissue regions and enhanced flow sensitivity in flow regions.
These and other problems in the prior art are overcome in large part by a system and method according to the present invention. An ultrasound system according to the present invention employs analysis of multiple frequency bands to distinguish between tissue and contrast agents. Since tissue is primarily a linear scatterer, with relatively minimal energy outside of the fundamental band and contrast agents significantly scatter more energy outside of the fundamental band, a selection unit can distinguish between tissue regions and contrast regions and apply different processing to each region.
In one embodiment, the selection unit employs the parameter B/A to distinguish between tissue and contrast agent signals.
In another embodiment, an ultrasound system according to the present invention employs a harmonic imaging filter on tissue only regions and implements frequency compounding filtering for contrast agent flow regions.
In another embodiment, tissue regions are displayed using grayscale maps and contrast agent regions are displayed in color.
In another embodiment, tissue regions are processed using echo processing techniques and displayed using grayscale maps while contrast agent regions are processed using power doppler and displayed using power doppler maps.
An ultrasound system according to the present invention can include a transmit/receive switch, an amplifier, analog-to-digital converters an RF data memory, frequency band selection means for selecting at least two frequency bands, and a region type selection unit. The region type selection unit can be used to determine if the data being received are from tissue or contrast agent regions. The frequency band selection means can be a first filter downconverter and second filter downconverter, one of which may be centered at a harmonic frequency while the other may be centered at the fundamental frequency. If the received signals are from tissue regions, a harmonic filter can be applied to the signal. If the signals are from contrast agent flow regions, both the harmonic filter and a fundamental filter can be used.
One method of generating 2 receive bands is a 2 pulse phase inversion method. This method includes generating and receiving two transmit pulses. The first pulse differs from the second by approximately 180 degrees phase difference, i.e., is an inverted pulse.
The first RF echo signal can be stored and applied to two down-converters. The first down-converter can be centered at the fundamental frequency. The second down-converter can be centered at a harmonic frequency. The two down-converter outputs can be filtered and stored.
A second RF echo signal can be obtained by sending the phase inverted pulse, which is added to the first RF echo signal. The RF summation result can be applied to the same down-converters used for the first transmit. In a selection unit, the ratio of the energy in the envelope-detected fundamental of the first received signal to the energy in the envelope-detected fundamental of the RF summation signal is obtained. This ratio can be used to detect bubble destruction, flow motion, non-symmetrical bubble response to compression and rarefaction pulses. In addition, the ratio of the harmonic output of RF summation to the fundamental of the first transmit is obtained and can be used to measure medium non-linearity, xe2x80x2B/Axe2x80x2. The two ratios can be sent to a decision maker circuit to determine if the signal is a tissue region or a contrast flow region. If the region is a tissue region, the harmonic output of RF summation is selected; if the region is a contrast flow region, the sum of outputs of both fundamental and harmonic from RF summation is used.
Another method of generating 2 receive bands is to apply 2 different bandpass filters to the received signal from a single transmit. Another method is to use phase inversion processing, transmitting 2 or more pulses that differ only in phase and summing the return signals in various ways to enhance or suppress various portions of the return signals.